基于数字散斑相关实验测量的材料构型力的计算方法

材料构型力学主要研究材料中的缺陷(夹杂、空穴、位错、裂纹、塑性区等)的构型(形状、尺寸和位置)改变时,所引起的系统自由能的变化。本研究将基于数字散斑相关技术,实验测量材料试件的位移场分布,随后通过材料构型力的定义式,计算求得弹塑性材料中缺陷构型力的分布。其方法概括如下:位移场通过数字图像相关技术测得;应变及位移梯度场利用三次样条拟合获得;线弹性材料应力通过简单线弹性本构方程获取,而塑性材料的表面应力场通过Ramberg-Osgood本构方程计算求得;弹塑性应变能密度分布则由应力-应变曲线数值积分获得。该方法对普通弹性材料或者弹塑性材料均适用,可以用于各种不同的缺陷及缺陷群的材料构型力测量。     

考虑损伤的内变量黏弹--黏塑性本构方程

基于Rice不可逆内变量热力学框架,在约束构型空间中讨论材料的蠕变损伤问题.通过给定具体的余能密度函数和内变量演化方程推导出考虑损伤的内变量黏弹--黏塑性本构方程.通过模型相似材料单轴蠕变加卸载试验对一维情况下的本构方程进行参数辨识和模型验证,本构方程能很好地描述黏弹性变形和各蠕变阶段.不同的蠕变阶段具有不同的能量耗散特点.受应力扰动后,不考虑损伤的材料系统能自发趋于热力学平衡态或稳定态.在考虑损伤的整个蠕变过程中,材料系统先趋于平衡态再背离平衡态发展.能量耗散率可作为材料系统热力学状态偏离平衡态的测度;能量耗散率的时间导数可用于表征系统的演化趋势;两者的域内积分值可作为结构长期稳定性的评价指标.     

考虑损伤的内变量黏弹-黏塑性本构方程

基于Rice 不可逆内变量热力学框架,在约束构型空间中讨论材料的蠕变损伤问题. 通过给定具体的余能密度函数和内变量演化方程推导出考虑损伤的内变量黏弹-黏塑性本构方程. 通过模型相似材料单轴蠕变加卸载试验对一维情况下的本构方程进行参数辨识和模型验证,本构方程能很好地描述黏弹性变形和各蠕变阶段.不同的蠕变阶段具有不同的能量耗散特点. 受应力扰动后,不考虑损伤的材料系统能自发趋于热力学平衡态或稳定态. 在考虑损伤的整个蠕变过程中,材料系统先趋于平衡态再背离平衡态发展. 能量耗散率可作为材料系统热力学状态偏离平衡态的测度;能量耗散率的时间导数可用于表征系统的演化趋势;两者的域内积分值可作为结构长期稳定性的评价指标.      

基于残余力向量的桁架梁损伤识别研究

定义残余力向量为结构的损伤指标,给出了根据结构特征值方程详细推导残余力向量的理论过程.并以简支桁架梁为研究对象,利用残余力向量进行桁架梁的局部损伤识别,成功识别出桁架梁的单一和多处局部损伤;同时考虑实际测量中存在噪声影响,引入了损伤定位指数,分析了噪声对损伤识别精度的影响.理论分析和数值仿真结果表明,残余力向量能有效识别桁架梁的局部中小损伤,且有较好的抗噪性.     

基于内变量理论的一种广义粘弹性本构方程

以Ｂｉｏｔ理论为基础，将弹性应变与非弹性尖变均取作状态为量，引入耗散势，建立了一种率相关非弹性本构关系及演变方程，进而导出一种广义的粘弹性本构方程。     

岩石材料单轴压缩损伤变量与纯剪切损伤变量间的关系

本文以岩石材料损伤应变能释放率为基础,利用不同的岩石屈服准则,探讨了岩石材料单轴压缩损伤变量与纯剪切损伤变量间的关系。借助于岩石材料单轴压缩或纯剪切中任一试验,即可求得两种受力状态下的损伤变量。     

混凝土损伤本构模型下梁承载力研究

为解决混凝土非线性特性难以描述的难题,通过MTS815.02岩石电液伺服系统,对圆柱体混凝土试件施加定围压比为0.2的围压,得出C25、C35、C45混凝土在＂理想无损伤＂状态下的应力-应变曲线;用Ramberg-Osgood公式描述其应力-应变关系,引入损伤变量演化方程,进而得出混凝土一维受压损伤本构模型。通过与实验数据的对比证明了模型的有效性和准确性;详细地分析了无初始损伤条件下配筋梁受弯破坏时的损伤演化过程;计算C25、C35、C45钢筋混凝土梁的受弯承载力。分析结果显示：随着混凝土强度由C25提高到C45,按此模型计算与按规范公式计算所得梁承载力的差异由1.9%升高至2.84%;下降段损伤累积量占总体损伤度的比例由46.26%升高至86.04%。研究表明,本文的分段曲线损伤模型可以较好地模拟梁混凝土的非线性受压过程,并能够给出损伤程度的预测。     

内聚力界面单元与复合材料的界面损伤分析

推导了一种基于内聚力模型无厚的界面单元,用来模拟复合材料纤维与基体之间的界面层．研究了纤维周期分布的复合材料受横向荷载时,在界面不同的强韧性条件下其界面损伤演化的规律和对复合材料整体性质的影响     

关于表征微裂纹型损伤的损伤变量的提出

本文分析了损伤变量的传统定义及其测量方法存在的问题，提出了以受损材料单位体积中所有微裂纹在某面上的投影面积的和作为表征材料在该面法向由这些微裂纹导致的损伤状态的损伤变量。     

考虑轴力作用时梁的弯曲损伤失效分析

对考虑轴力作用时梁的弯曲损伤进行了分析,推导了其损伤分析的基本方程,分析表明,中性轴随损伤的发展而偏移,有向上、向下和保持不动三种情况,其偏移的方向和范围完全由比例加载系数决定,并且在特定的比例加载系数时,中性轴能保持不动,最后获得了梁在弯曲损伤时的中性轴起始偏移点、极限偏移点和极限载荷的计算公式,以及它们的简化计算公式;讨论了损伤极限状态时轴力和弯矩的交互作用曲线;轴力对弯曲损伤的影响较为显著,有轴力作用时应尽可能考虑其影响。     

On configurational forces in multiplicative elastoplasticity

The main goal of this work consists in the elaboration of the material or rather configurational mechanics in the context of multiplicative elastoplasticity. This nowadays well-established approach, which is inherently related to the concept of a material isomorphism or in other words to a local rearrangement, is adopted as a paradigm for the general modelling of finite inelasticity. The overall motion in space is throughout assumed to be compatible and sufficiently smooth. According to the underlying configurations, namely the material and the spatial configuration as well as what we call the intermediate configuration, different representations of balance of linear momentum are set up for the static case. The underlying flux terms are thereby identified as stress tensors of Piola and Cauchy type and are assumed to derive from a free energy density function, thus taking hyperelastic formats. Moreover, the incorporated source terms, namely the configurational volume forces, are identified by comparison arguments. These quantities include gradients of distortions as well as dislocation density tensors. In particular those dislocation density tensors related to the elastic or plastic distortion do not vanish due to the general incompatibility of the intermediate configuration. As a result, configurational volume forces which are settled in the intermediate configuration embody non-vanishing dislocation density tensors while their material counterparts directly incorporate non-vanishing gradients of distortions. This fundamental property enables us to recover the celebrated Peach–Koehler force for finite inelasticity, acting on a single dislocation, from the intermediate configuration volume forces.     

On configurational inertial forces at a phase interface

The evolution equations for a phase interface are discussed. Interfacial structure is neglected, as are thermal and compositional variations. The focus is on a new treatment of the inertial forces at the interface.     

Generalized thermomechanics with dual internal variables

The formal structure of generalized continuum theories is recovered by means of the extension of canonical thermomechanics with dual weakly non-local internal variables. The canonical thermomechanics provides the best framework for such generalization. The Cosserat, micromorphic, and second gradient elasticity theory are considered as examples of the obtained formalization.     

Failure theory via the concept of material configurational forces associated with the M-integral

A new failure theory based on the material configuration forces associated with the invariant M-integral is proposed to describe the content and evolution of the multi-defects localized in the body. The physical interpretation of the global M-integral is as the sum of the local energy release rate due to the self-similar expansion for each specific defect. It does provide an effective measure for the evaluation of damage level. It is found that the unique parameter of the M-integral cannot be used as a unified failure criterion to predict the damage evolution and the final failure due to the major obstacle that the critical value of the M-integral is not a problem-invariant constant and shows an apparent defect configuration-dependence. Consequently, a new failure parameter referred as the configurational damage parameter (abbreviated as Π-parameter) is proposed by the appropriate formulation via the M-integral, the remote uni-axial load, and the inner variable of the damaged area. A series of numerical examples are carried out to demonstrate that the critical value of Π-parameter is a material constant regardless of defect configurations. Furthermore, it is performed to validate the applicability of the Π-parameter as a failure criterion to predict the final failure of the locally damaged materials. Finally, a protocol of experimental measurement of the Π-parameter is proposed by method of digital image correlation to facilitate the wide application of the new failure criterion. It is concluded that the present failure theory via the configurational forces associated with the M-integral provides some outside variable features and has the advantage of predicting the structural integrity of damaged materials containing the locally distributed defects.     

Investigation on thermomechanical fracture in the framework of configurational forces

A configurational force approach is developed for providing a fresh look onto classical aspects of thermomechanical fracture. The theoretical framework is based on the finite deformation and makes no restrictions on the material response. The integral form of configurational force balance at the crack tip is constructed, and the concentrated configurational body force is decomposed into the inertial and internal parts. The energy release rate is evaluated through the generalized second law of thermodynamics applicable to configurational force system. The theoretical investigation shows that the negative of the projection of the internal configurational force concentrated at the crack tip along the direction of crack propagation plays the role of the energy release rate and acts directly in response to crack propagation. This finding enables us to deal with the thermomechanical fracture problems in material space.